1. Field of the Invention
The present invention relates to a process for preventing undesired polymerization in a mixture containing ethylenically unsaturated compounds, in particular in the isolation of ethylenically unsaturated compounds from a mixture containing them by distillation.
2. Description of the Background
It is known that many unsaturated compounds tend to undergo generally free radical polymerization when the temperature is increased. For example, vinylaromatic compounds, such as styrene or xcex1-methylstyrene, have to be stabilized with suitable compounds to prevent premature polymerization in the purification of the industrially obtained crude products by distillation. Usually, stabilizers or polymerization inhibitors are added to the crude products to be distilled, before or during the purification step. In spite of this measure, a certain amount of oligomers or polymers is obtained. In specific cases, especially when operating faults occur, complete polymerization of the monomers present or of the monomer mixture can take place during the purification or distillation. Costs are incurred as a result, owing to the extensive cleaning effort and loss of production.
The ethylenically unsaturated compounds obtained in pure form must furthermore be protected from undesired premature polymerization during storage or handling, for example during derivatizing.
USSR patents SU-1027150, SU-1558888 and SU-1139722 describe the stabilization of styrene by using nitroxyl or bisnitroxyl compounds.
WO-96/16921 discloses mixtures of vinylaromatic compounds with sterically hindered nitroxyl compounds, which are activated by traces of oxygen.
JP Hei 1-165534 discloses piperidyloxy derivatives as polymerization inhibitors for styrene.
U.S. Pat. No. 5,254,760 and DE-19622498 describe mixtures of nitroxyl and nitro compounds for stabilizing vinylaromatic compounds during purification or distillation.
DE 19651307 describes mixtures which contain vinyl-containing compounds, such as styrene, and a mixture of an N-oxyl compound and an iron compound, which mixture inhibits premature polymerization. The mixtures are effectively stabilized against premature polymerization during purification or distillation.
To ensure the sufficient stabilization, the nitroxyl radicals must be present in a specific minimum concentration in the mixtures containing ethylenically unsaturated compounds. As a result of scavenging reactions with spontaneously forming radicals, a certain amount of nitroxyl radicals is continuously consumed. The rate of consumption depends on external variables, such as the temperature, the presence of free radical initiators, the admittance of oxygen, etc. These variables can sometimes fluctuate unpredictably. To ensure sufficient stabilization even under unfavorable conditions, the nitroxyl radical must therefore be added in a more or less large excess to the mixtures to be stabilized. Since nitroxyl radicals are comparatively expensive polymerization inhibitors, the required amount of nitroxyl radicals for stabilization during purification and/or handling constitutes a cost factor which is not negligible. It is therefore desirable to keep the excess in the metering of nitroxyl radicals as small as possible or to avoid such excess.
It is an object of the present invention to provide a process for preventing undesired polymerization in a mixture containing ethylenically unsaturated compounds, in particular in the isolation of ethylenically unsaturated compounds by distillation from a mixture containing them, in which process the stabilizer is used very efficiently.
We have found that this object is achieved if the concentration of the N-oxyl radicals having at least one unpaired electron can be electronically determined in a comparatively simple manner so that an addition of stabilizer can be effectively controlled.
The present invention accordingly relates to a process for preventing undesired polymerization in a mixture containing ethylenically unsaturated compounds by maintaining an effective concentration of a stabilizer system which comprises N-oxyl radicals, wherein
(i) an electronic signal which correlates with the concentration of the N-oxyl radicals in the mixture is obtained periodically or continuously,
(ii) the electronic signal is compared with a reference value and
(iii) an addition of stabilizer system to the mixture is controlled according to the comparison.
The present invention also relates to a process for isolating ethylenically unsaturated compounds from a mixture containing them by distillation of the mixture while maintaining an effective concentration of a stabilizer system in the mixture which comprises free N-oxyl radicals, wherein
(i) an electronic signal which correlates with the concentration of the N-oxyl radicals in the mixture is obtained periodically or continuously,
(ii) the electronic signal is compared with a reference value and
(iii) an addition of the stabilizer system to the mixture is controlled according to the comparison.